docs: def _get_site_type_dummy_variables(self, sites):
          """
          Get site type dummy variables, which classified the sites into
          different site classes based on the shear wave velocity in the
          upper 30 m (Vs30) according to the EC8 (CEN 2003):
          class A: Vs30 > 800 m/s
          class B: Vs30 = 360 - 800 m/s
          class C*: Vs30 = 180 - 360 m/s
          class D: Vs30 < 180 m/s
          *Not computed by this GMPE
          """
          ssa = np.zeros(len(sites.vs30))
          ssb = np.zeros(len(sites.vs30))
          ssd = np.zeros(len(sites.vs30))
          # Class D; Vs30 < 180 m/s.
          idx = (sites.vs30 < 180.0)
          ssd[idx] = 1.0
          # Class B; 360 m/s <= Vs30 <= 800 m/s.
          idx = (sites.vs30 >= 360.0) & (sites.vs30 < 800.0)
          ssb[idx] = 1.0
          # Class A; Vs30 > 800 m/s.
          idx = (sites.vs30 >= 800.0)
          ssa[idx] = 1.0
  
          for value in sites.vs30:
              if 180 <= value < 360:
                  raise Exception(
                      'GMPE does not consider site class C (Vs30 = 180-360 m/s)')
  
          return ssa, ssb, ssd